Semicrystallized ground coats and enameled articles manufactured therefrom



United States Patent 3,458,344 SEMICRYSTALLIZED GROUND COATS ANDENAMELED ARTICLES MANUFACTURED THEREFROM John R. Little, Fairport, andElbert A. Sanford, Rochester, N.Y., assignors to Sybron Corporation, acorporation of New York No Drawing. Original application Nov. 20, 1964,Ser. No. 412,850. Divided and this application Sept. 28, 1967, Ser. No.683,050

Int. Cl. 344d 1/14; C2311 /00 US. Cl. 117-70 4 Claims ABSTRACT OF THEDISCLOSURE Disclosed is a glass-metal article wherein the metalsubstrate is mild steel and the glass coating includes a partlycrystallized ground coat consisting essentially of 5-20% by weight of amixture of P 0 and B 0 the weight ratio of P 0 and B 0 being within therange of 1:3 and 2:1, 420% by weight of alkaline earth oxides, 9-23% byweight of alkali oxides, 0-20% by weight of TiO 0-3% by weight of anadherence oxide and 45-65% by weight of SiO This is a divisional ofapplication Ser. No. 412,850 filed Nov. 20, 1964.

This invention relates generally to improved ground coat compositionsfor use in the manufacture of glass or vitreous enamel coated metals andmore particularly relates to improved ground coat compositions for usein systems comprising mild or low-carbon steels with partiallycrystallized cover coats thereover.

Glass coated metals are widely used in the chemical and pharmaceuticalindustries as construction materials for applications that require acombination of strength and exceptional corrosion resistance.

To assure good adhesion between the base metal and the glass coatings,it is common to pretreat the surface as by sandblasting, degreasing,decarburization, etc. A further expedient, well known in the art, is theapplication of a so-called ground coat enamel upon the surface of themetal. Ground coats are specially formulated to enhance uniform coatingover the base metal and to form a strong bond between the surface orcover coat and the base metal. These ground coats are characterized bylow surface tension, the ability to wet the metal surface and capacityto dissolve oxides that form when the composite article is heated. Inaddition to improved adherence, the ground coat also must besubstantially free of enameling defects such as pin holes, and should beeffective in controlling or contributing to the properties of the glassmetal composite. After the aforesaid ground coat has been applied, oneor more cover coats are applied thereover. These cover coats areformulated to provide the surface characteristics required for theparticular application.

Recently, partially-crystallized glass cover coats have been developedfor use at temperatures in excess of 1000 F. Although many ground coatformulations are presently available, they were developed for use insystems where operating temperatures seldom exceeded 500 F. and are notsuitable for use at significantly higher temperatures, because theyexhibit softening temperatures, i.e., the temperature at which flow willoccur, of less than 1000 F. Because of the relatively low softeningpoints of these conventional ground coats, they are susceptible tothermal degradation when used at the normal operating temperatures ofthese newly developed high temperature surface or cover coatings. Thisdegradation results in a premature failure of the glass-metal composite.

In addition to better thermal properties, these new cover coats exhibitphysical and mechanical properties, such as resistance to thermal shockand impact, that far exceed the properties of conventional ground coats.Thus when conventional ground coats are used with these new highstrength cover coats, the properties of the cover coat cannot be fullyutilized because of the weaknesses of the ground coat. The primaryobject of this invention is to provide ground coats that will coverlow-crabon or mild steels, crystallize in situ and have increasedphysical and thermal properties compared to conventional amorphousground coats used for mild steels.

A ground coat suitable for use in a glass-metal system must also exhibitcertain viscosity characteristics. During the initial firing, it isimportant that the glass flow rapidly and easily over the metalsubstrate to assure uniform coating and an intimate contact therewiththat will facilitate the formation of a strong bond between the groundcoat and the metal substrate. A viscosity at fusion of 1000 to 10,000poises has been found suitable, and the preferred viscosity at fusion isabout 3000 poises. However, to provide desirable physical properties,such as thermal shock resistance, impact resistance and thermalstability, it is necessary to strengthen the ground coat by making itmore refractory. To solve the above problems, it has been proposed toincrease the refractoriness of ground coats by adding refractory oxidesto the presently used amorphous compositions. However, we have foundthat the refractory coatings produced in this manner exhibit erraticphysical properties when used in glass-metal systems. It has also beenproposed to vary the composition of the ground coat to achieve a highsoftening point. Although such variation may produce more refractorycoatings, the resultant ground coats exhibit high viscosity at normalfiring temperatures and therefore require excessively high firingtemperatures to produce the low viscosity necessary to provide smootheven coatings. It is thus another object of this invention to provide aground coat composition that exhibits the combined properties of lowviscosity during the initial firing and increased refractoriness in thefinal product.

Because of the higher operating temperatures of systems including thesenew cover coats, it is necessary to provide ground coats that havesufficient thermal stability to inhibit excessive thermal degradationduring use. It is accordingly an object of this invention to provideground coats exhibiting thermal stability when exposed to temperaturesin excess of 1000 F. for several hundred hours.

A successful ground coat must provide good adherence between the basemetal and the cover coat. Furthermore it is well known, within theenameling art, that the ground coat must be able to dissolve the scaleformed on the base metal during firing before good adherence can beobtained. Scale solution is particularly important when the substrate tobe enameled is a mild or carbon steel (steel containing up to 0.25%carbon) in contrast to a high alloy material, as mild steels formsubstantially more oxide scale. In a co-pending application we haveproposed the crystallization in situ of a ground coat as a means forimproving the physical properties of a glass-metal composite including ametal substrate in which the major elements are cobalt, chromium,nickel, iron, or combinations thereof. However, in attempting toformulate crystallizable ground coats for use on mild steel substrateswe found that the proposed compositions for high-alloy materials werenot suitable as they could not adequately dissolve the scale.Furthermore we found that the conventional practice, used in formulatingconventional amorphous ground coats for mild steels, of adding B wasunsuitable, as in amounts necessary for scale solution B 0 severelylimited or prevented crystal growth and generally lowered the thermalexpansivity of the ground coat, resulting in undesirably high shearingstresses at the ground coat metal interface.

In contrast, we have found that by the inclusion of P 0 and B 0 within acritical ratio that the combined goals of scale solution andcrystallization are achieved. Accordingly, it is a basic object of thisinvention to provide P O -containing ground coat compositions capable ofdissolving scale formed on mild steel substrates, having thermalexpansivities compatible with mild steel substrates, and exhibitingcrystallization properties'suitable for enameling purposes.

To function properly as a composite article, the thermal expansivity ofthe individual glass coats must be carefully controlled to obtainmaximum physical properties. Glass or vitreous enamel is known to bestronger in compression than in tension, thus thermal expansion of theground coat should not be greater than that of the base metal. However,even though compression is desirable, the magnitude of such compressionshould be controlled. Where the thermal expansivity of the ground coatis the same as or only slightly less than the base metal, interfacialshearing stresses are reduced. Furthermore, since the ground coat issandwiched between the base metal and the cover coat these sameconditions dictate that the cover coat thermal expansivity should be thesame as or slightly less than the ground coat. This lowers theinterfacial stress and thereby minimizes the possibility of violentfailure when the composite is exposed to thermal shock. Thermalexpansion characteristics are also important in preventing catastrophiclosses on impact. A ground coat suitable for use with mild steels shouldhave a thermal expansion coeflicient of 47 l0 in./in./ F. from 80 to 800F. Therefore, a specific object of this invention is to provide a groundcoat composition exhibiting thermal expansion characteristics falling inthis range, suitable for use with mild steel substrates.

As previously stated, a ground coat must be substantially free ofenameling defects such as pinholes, blisters, etc. It is yet anotherobject of this invention to provide ground coat compositions that areformulated to effect the above stated objects of high temperaturestrength and thermal stability, yet also be substantially free ofenameling defect.

Although these coatings are specifically directed to achieve the dualfunction of crystallization and solution of relatively large amounts ofoxide scale, they may be used on base metals such as Inconel, stainlesssteels and the Hastelloys. This is because these metals do not form asmuch oxide as mild steels, and thus the disclosed ground coats canadequately dissolve the scale formed by them. However, these coatingscrystallize less than others, such as disclosed in our co-pendingapplication, and therefore are less suitable for use on alloy substratesin high temperature operations.

It is a further object of this invention to provide a composite articlecomprising the novel ground coats disclosed herein.

Other objects and advantages of this invention Will become apparent whenread in conjunction with the following description of our invention.

Briefly stated, we have found that by a partial substitution of P 0 forB 0 ground coats may be formulated that not only dissolve iron oxides,but also crystallize in a satisfactory manner. Furthermore, the additionof P 0 results in ground coats exhibiting significantly increasedthermal expansion and thus reduces undesirable interfacial stresses. Inaddition, we have found that the properties of said P 0 containingcrystallizable ground coats may be adjusted by the use of oxides such asBaO, CaO, MgO,

4 TiO etc. in amounts which cause them to take part in thecrystallization.

In the application of enamel coatings to metal substrates, it isrecognized that application to mild or low carbon steels presentsdifliculties that are not present in high alloy materials. These mild orlow carbon steels tend todevelop relatively large quantities of scaleduring heat treatment and processing. In most carbon steel ground coats,large amounts (8-15 of B 0 are used to aid in scale solution. However, B0 cannot be used in such quantities in conventional ground coat glasseswhen crystallization is also desired.

In our ground coats P 0 and B 0 are both necessary and the amountspresent must be maintained within critical limits. Too much B 0 inhibitscrystallization and decreases the thermal expansion. However, too much P0 will prevent smelting of the glass. B 0 is a good flux and compensatesfor the effect of the P 0 on smelting, and P 0 compensates for the lossof scale solution properties caused by the decreased B 0 content. Thetotal content of B 0 and P 0 cannot exceed 20% of the preferred ratio ofP 0 B 0 is 1:1. However, compositions within the range of 1:3 to 2:1 areuseful for the practice of our invention.

In the glass compositions set forth in this disclosure SiO is theprinciple glass former. Amounts significantly greater than 65% willcause the coating to be too refractory, while amounts significantly lessthan 45% result in too fluid a coating.

BaO, MgO, and CaO are also crystal formers. In addition they contributeto thermal expansion, fusion, and smelting properties. Regulation ofthese oxides is necessary to control the rate of crystal growth.

The alkali oxides, Na O, U 0, and K 0 are essential aids in adjustingthermal expansion, smeltability and particularly fusion temperatures.The glass compositions would not be useful for ground coats in theabsence of these constituents as the firing temperatures would beexcessively high. However, the use of Li O should be carefully regulatedas too much seems to promote enameling. defects.

TiO contributes to crystal formation, however, below 5% it is noteffective, and in amounts greater than 20% pure Ti0 which is of noparticular value in these glasses, precipitates. While some ground coatshave been prepared without TiO best results thus far have included TiOAdherence oxides, such as C0 0 NiO, MnO etc. a d in the chemical bondingof the ground coat to the base metal.

Amounts up to 3% can be used for this purpose. Greater amounts are notharmful, but are of no particular advantage.

,Various other oxides such as Ce0 SrO, ZnO, ZrO etc. could be used toaffect certain properties of the disclosed glasses. However, amounts inexcess of 5% would probably significantly alter the properties.

Specifically, we have found that compositions within the followingranges are suitable for use as partially crystalized ground coats onmild steel substrates.

The total of P 05 and B203 not to exceed 20 and the ratio of P 05 andB203 being between 1: 3 and 9 Z The following examples will furtherillustrate the pract1ce of the subject invention:

EXAMPLE I Raw batch materials were dry mixed in amounts cal- 5 culatedto provide a glass having a weight percent ana' lysis of The resultingbatch was fused at a temperature between 2400-2800 F., and the moltenglass was quenched directly into cold water to produce a frit.

The frit was milled in water with appropriate mill additions, includingclay, sodium nitrite, potasium chloride, etc., to produce a slip. Theslip was spray applied to a low-carbon steel plate, the surface of whichhad previously been blasted with A1 grit. After drying to remove excesswater the article was then heated to a temperature between 1600-1850 F.to fuse the glass. The viscosity at fusion was about 3000 poises.

After fusion of the ground coat a crystallizable cover coat was appliedthereover.

Finally the composite article was heat treated at about 1450" F.,thereby partially crystallizing both the ground coat and the cover coat.

A comparison of the physical properties of a composite formed accordingto Example I and an article similarly treated but including an amorphousborosilicate ground coat follows:

THE RMAL SHO CK Raw batch materials were dry mixed in amounts calculatedto provide a glass having a weight percent analysis of Percent SiO 51.9P 0 4.8 B 0 4.8 BaO 4.8

Na O 14. K 0 4.8 Ti0 14.3 Adherence oxides 1.0

The resulting batch was smelted at a temperature between 2500 and 2700F., frited, and milled with appropriate mill additions. The resultingslip was applied to a low-carbon steel plate to build a thickness of 68mils and then fused at a temperature between 1750 and 1800 F.

After fusion of the ground coat a crystallizable cover coat was appliedthereover and the composite article was heat treated at a temperature ofabout 1440 F. Upon such treatment a stuffed barium silicatesolid-solution type crystal was found to be predominant.

The resulting composite article exhibited improved thermal shockproperties compared to articles having an amorphous borosilicate typeground coat. The failure temperature was increased 250300 F. and thetemperature at which the first glass loss occurs was increased ISO-200F. The thermal expansivity was considerably higher than conventionalborosilicate ground coats, thus interfacial shear and the tendencytoward violent or premature failure upon exposure to impact or thermalshock was reduced.

Impact resistance properties were determined by subjecting the articleafter impacting to an applied voltage of 10,000 volts and determiningthe extent of electrical contact. The subject article could be exposedto 20% more impact force before the first electrical contact between theprobe and base metal occurred. Furthermore, a 30% increase in impactresistance was noted before the first glass loss occured.

The term partial crystallization as used herein means the formation ofcrystals in situ within an amorphous matrix. Wherever the term groundcoat is used it should be understood to include the practice of forminga thick ground coat by the successive application of a plurality of thincoats prior to the application of the final surface coat.

The term cover coat should also be construed to include a plurality ofglass coats, each having the composition of a cover coat as contrastedto that of a ground coat. A suitable crystallizable cover coatcomposition is as follows:

With respect to the method of application of the glass coats, althoughwe have described a spraying method, this is not considered alimitation. Any of the commonly used methods are within the scope of ourinvention.

It will thus be seen that this invention attains its stated objects. Itprovides a crystallizable ground coat suitable for use with low carbonor mild steels and having physical and thermal properties that enhancecomposite articles including high-strength high temperature cover coats.Further, this invention provides ground coats with the combinedproperties of viscosity at fusion suitable for easy flow over metallicsubstrates and increased refractoriness in the final product. Theseground coats also permit the obtaining of thermal expansioncharacteristic suitable for use with mild or low carbon substrates,without sacrificing scale solution properties or ease ofcrystallization. In addition, ground coats according to this inventionare substantially free of enameling defects and thus provide heretoforeunavailable high-strength corrosion-resistant composite articlesincluding mild steel or low carbon substrates, rather than expansivealloy substrates.

While we have shown and described the preferred methods and compositionsof our invention, it will be apparent that those having the benefit ofour disclosure will be able to make various changes and modifications,therein, without departing from the spirit of our invention as set forthin the appended claims.

We claim:

1. A glass metal composite comprising a combination: a metal substrate;a layer of partially crystallized ground coat enamel fused to thesurface of said metal substrate; at least one cover coat of corrosionresistant enamel covering said ground coat enamel; said ground coatenamel consisting essentially of 520% by weight of a mixture of P 0 andE 0 the weight ratio of P 0 and B 0 being within the range of about 1:3and 2: 1, 420% by weight of alkaline earth oxide, 923% by weight ofalkali oxide, 020% by weight of T10 03% by weight of an adherence oxideand 4565% by weight of SiO whereby a smooth, continuous, substantiallypin-holefree enamel coated object is provided.

2. The article of claim 1 wherein said metallic substrate is a mildsteel.

3. The article of claim 1 wherein said cover coat is partiallycrystallized.

4. The article of claim 1 wherein said ground coat enamel contains P 0and B 0 in a substantially 1:1 weight percent ratio.

Commons et a1. 106-48 Judd 106-48 Sanford et a1. 117-70 X Van Dolah10648 Nelson 117-70 X ALFRED L. LEAVI'IT, Primary Examiner C. K.WEIFFENBACH, Assistant Examiner U.S. Cl. X.R.

